The process of extracting methane gas from carbon compound bearing material such as animal waste by anaerobic bacterial digestion is well known and has received considerable, recent attention because of the energy crisis. As a source of fuel gas and fertilizer the process has attraction both for small, farm unit operation and larger feedlot applications where ground water pollution is a persistent problem.
In a conventional application livestock waste is collected and diluted with water to form a slurry. The slurry is pumped into an air-tight container or digester. The slurry in the digester is heated to maintain it at a temperature of the order of 37.degree. C. and the pH is adjusted and maintained within the range of 7.5 to 8.5. Under these conditions, the process of anaerobic fermentation ultimately leads to the evolution of methane, carbon dioxide and hydrogen, with traces of other gases. A sludge, useful as fertilizer remains as a residue.
During the digestion process methane production is limited by several inhibiting factors. One of these is the gradual formation of a scum at the surface of the digesting slurry. This scum and undigested solid particles accumulated in the digester require, eventually, that the gas producing operation be halted and the digester be cleaned out. Another inhibiting factor, present initially after loading the digester is the oxygen carried in with the slurry. Until this initial oxygen is transformed to carbon dioxide by aerobic bacteria, production of methane by the anaerobic bacteria cannot start. Rapid withdrawal of this initial oxygen from the system can thus decrease the detention time of the raw material in the digester. A further and primary inhibiting factor results from the concentration of dissolved ammonia reaching a toxic level in the digester. Since urine contributes approximately two-thirds of the ammonia present in the digester slurry, conventional methane generator plants have had to exclude the nitrogen-rich urine or increase the size of the digester vessel so that the concentration of ammonia can be decreased by increased dilution with water. Both alternatives have obvious disadvantages.
Anaerobic bacterial digestion processes are disclosed in U.S. Pat. Nos. 3,383,309 and 2,198,737. These attempt to deal with the digestion inhibiting factors by introducing hydrogen gas and recycled digester gases into the slurry in the digester (U.S. Pat. No. 3,383,309) and by reducing the formation of scum through mechanically agitation of the slurry (U.S. Pat. No. 2,198,737).
The process of the present invention differs from conventional methods of methane extraction primarily in that, during the digestion process, a sub-atmospheric pressure is maintained in the digester. This provides several unexpected, efficiency-enhancing results.
As presently understood, the sub-atmospheric pressure above the slurry level in the digester vessel stimulates bacterial action by drawing gas out of the slurry as it is generated, insuring constant anaerobic fermentation and accelerated decomposition of the raw waste. The dissolved ammonia in the slurry, which slows or inhibits methane production in the conventional methane generator plant, is rapidly drawn from the slurry, as free ammonia with the methane gas. The inhibiting effect of the ammonia is thus minimized. Further the eruption of the gas from the slurry due to the accelerated rate of gas evolution tends to break up scum formation. The result is more rapid production of methane gas and makes possible a reduction in the detention time of the slurry in the digester of the order of sixty-five percent, that is, in a typical installation, a reduction from twenty-eight days to ten days. The ammonia, extracted from the output side of the system, represents a valuable by-product.